Phase separation analysis of bulk heterojunctions in small-molecule organic solar cells using zinc-phthalocyanine and C sub(60)

To achieve efficient organic solar cells, donor and acceptor molecules are mixed in the photo-active layer to form a so-called bulk heterojunction. Due to molecular interactions, a certain degree of phase separation between donor and acceptor domains arises, which is necessary to achieve efficient charge extraction within the absorber layer. However, the mechanism that induces the phase separation is not fully understood and gaining detailed information about the molecular arrangement within these blend layers is quite challenging. We show that grazing incidence x-ray diffraction, combined with variable angle spectroscopic ellipsometry is a suitable way to investigate the molecular structure of blend layers in detail, consisting of a mixture of zincphthalocyanine (ZnPc) and C[sub 60]. However, grazing incidence x-ray diffraction measurements reveal that ZnPc and C[sub 60] already form separate domains in unheated ZnPc:C[sub 60] blends, which provide fill factors close to 50 in the corresponding solar cells.